1. Field of the Invention
The present invention relates to a method for polymerizing alpha-olefins, a catalyst for such a polymerization method and a method for producing such a catalyst. A particular aspect of the present invention relates to a method for preparing a catalyst which produces linear low density polyethylene (LLDPE) having a relatively narrow molecular weight distribution, as evidenced by relatively low values of melt flow ratios (MFR), suitable for film and injection molding applications.
2. Description of the Prior Art
Linear low density polyethylene polymers possess properties which distinguish them from other polyethylene polymers, such as homopolymers of polyethylene. Certain of these properties are described in Anderson et al, U.S. Pat. No. 4,076,698.
Karol et al, U.S. Pat. No. 4,302,566, describe a process for producing certain linear low density polyethylene polymers in a gas phase, fluid bed reactor.
U.S. Pat. Nos. 4,173,547, to Graff, 3,787,384, to Stevens et al, 4,148,754, to Strobe et al, and 4,063,009, to Ziegler deceased, each describe various polymerization processes suitable for producing forms of polyethylene other than linear low density polyethylene, per se.
U.S. Pat. No. 4,173,547, to Graff describes a supported catalyst obtained by treating a support with both an organoaluminum compound and an organomagnesium compound followed by contacting this treated support with a tetravalent titanium compound.
U.S. Pat. Nos. 3,787,384, to Stevens et al, and 4,148,754, to Strobel et al, describe a catalyst prepared by first reacting a support (e.g., silica containing reactive hydroxyl groups) with an organomagnesium compound (e.g., a Grignard reagent) and then combining this reacted support with a tetravalent titanium compound. According to the teachings of both of these patents, no unreacted organomagnesium compound is present when the reacted support is contacted with the tetravalent titanium compound.
Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, describe a catalyst which is the reaction product of an organomagnesium compound (e.g., an alkylmagnesium halide) with a tetravalent titanium compound. The reaction of the organomagnesium compound with the tetravalent titanium compound takes place in the absence of a support material.
A vanadium-containing catalyst, used in conjunction with triisobutylaluminum as a co-catalyst, is disclosed by W. L. Carrick et al in Journal of American Chemical Society, Volume 82, page 1502 (1960) and Volume 83, page 2654 (1961).
Nowlin et al, U.S. Pat. No. 4,481,301, the entire contents of which are incorporated herein by reference, disclose a supported alpha-olefin polymerization catalyst composition prepared by reacting a support containing OH groups with a stoichiometric excess of an organomagnesium composition, with respect to the OH groups content, and then reacting the product with a tetravalent titanium compound.
It is a primary object of the present invention to prepare a high activity catalyst for the polymerization of alpha-olefins which yields products of a relatively narrow molecular weight distribution suitable for films and injection molding applications.
It is an additional object of the present invention to provide a catalytic process for polymerizing alpha-olefins which yields linear low density polyethylene of a relatively narrow molecular weight distribution.